1. Field of the Invention
The invention relates to a desulfurizing composition and method for desulfurizing molten pig iron, cast iron and malleable iron.
2. Description of the Prior Art
Steelmakers generally desire to have a minimum amount of sulfur in the steel they produce, as well as in the molten iron from which the steel is made. Presently, molten iron from the blast furnace is desulfurized by the injection of a suitable reagent with a carrier gas, usually nitrogen. One widely used desulfurizing reagent is a mixture of particulate lime and particulate magnesium. Although this reagent performs well as a desulfurizer, steelmakers have been seeking alternative reagents. This search has been prompted by the facts that magnesium lime reagents are flammable and that metallic magnesium, which may be 90% of this mix, is quite costly.
Several people have proposed to substitute metallic aluminum or metallic aluminum and alumina for the magnesium. Mitsuo et al. in U.S. Pat. No. 4,374,664 assigned to Nippon Steel disclose a process for desulfurizing molten pig iron in which powdered aluminum and lime or powdered aluminum, alumina and lime are injected into molten pig iron. Although this composition provides adequate desulfurization, metallic aluminum is also quite costly.
In U.S. Pat. No. 5,021,086 there is disclosed an iron desulfurization additive containing a granular mixture of metallic magnesium, calcium oxide and a small amount of hydrocarbon containing material which provide a volatile gas producing component to the mixture. The patent teaches that the hydrocarbon constituent improves the desulfurization of the magnesium-lime mixture by increasing the surface area of the magnesium-lime agglomerations. At high operating temperatures found in molten iron, the hydrocarbon constituent forms a gas which breaks down the magnesium-lime agglomerations. This desulfurizer is relatively expensive.
In the summer of 1995 Reactive Metals & Alloys Corporation, one of the assignees of the present application, tested a reagent at an integrated steel plant in the United States which contained 65% lime, 27% aluminum, 6% fluorspar and 2% hydrocarbons. This reagent was injected at the rate of from 1.5 to 6 pounds reagent per ton of molten metal and resulted in removal of from about 0.003% to about 0.02% sulfur. Those observing the trial were disappointed because the low sulfur removal and the cost of pure aluminum resulted in an unacceptable cost per point of sulfur removed. Fluorspar could have contributed to the low sulfur removal.
Consequently, there is a need for a low cost desulfurization composition that can be used in molten iron from a blast furnace which will remove at least 40% and preferably near 100% of the sulfur present in the molten iron.
The following reaction: ##EQU1## is generally recognized as underlying all the lime-based desulfurizing processes. Provided an excellent "home" is continuously provided in situ for the liberated oxygen atom, the reaction can be completed towards residual sulfurs in molten metals below 1 ppm S if required.
There are many elements which will react with the free oxygen if those elements are present in the hot metal. At typical 2400.degree. F. temperatures of hot metal to be desulfurized, free energies of formation of the oxide compounds show the following preference for the effectiveness of such elements for the liberated oxygen atom expressed as .DELTA.G.degree.f in BTU's per pound-mole of O.sub.2 gas.
______________________________________ 440 Scandium, Sc metal 410-420 Heavy Lanthanides Yttrium: Y, Dy, Ho, Er, Tm, Lu 405 Calcium, Ca metal alloys and compounds 390 Strontium, Sr metal 380-395 Light Lanthanides: La, Ce, Nd, Pr . . . Gd 375 Beryllium, Be metal 355 Barium, Ba metal 350 Magnesium, Mg metal and alloys 340 Zirconium, Zr metal and alloys 335 Aluminum, Al metal and alloys 280-325 Titanium, Ti metal and alloys 255 Silicon, Si metal and alloys ______________________________________
Almost all of these elements have been rejected as the deoxidizing additive for hot metal desulfurization because of their cost. Scandium for example costs about $10,000.00 per pound. Beryllium and barium are toxic as well as expensive. Only calcium and magnesium have been used extensively. Calcium metal and calcium silicon are too expensive. Calcium carbide, CaC.sub.2, is extensively used worldwide for molten pig iron desulfurization. However, because of its price and lack of complete molecular splitting at 2400.degree. F., calcium carbide will lose its competitiveness with our composition. There are also safety concerns about using calcium carbide. Pure magnesium and magnesium alloys have been used because they are less expensive than the alternatives, but they are still costly. Silicon metal and alloys are economical, but tend to form SiO.sub.2 which forms solid envelopes of dicalcium silicate, 2CaO--SiO.sub.2, blocking the process.
From a strictly thermodynamic equilibrium consideration viewpoint, the above list indicates that aluminum which is close enough to magnesium in free energy of formation should perform almost as well as magnesium. Indeed, literature going back several decades, for and U.S. Pat. No. 4,374,664 to Nippon Steel clearly confirm that aluminum and aluminum alloys have been given extensive and serious experimentation as critical additive to lime for hot metal desulfurization and have performed to some extent.
If an aluminum containing agent is used, it is of paramount importance that the highest possible concentration of aluminum be present at the same location as where solid lime encounters sulfur atoms dissolved in the molten metal being treated. Thus, pretreatments by aluminum have to be inferior, kinetically and economically because aluminum tends to be strongly depleted locally, stopping the reaction. Also, this indicates that it is redundant and uneconomical to provide excess aluminum content in the hot metal before, during or even after lime injection. That observation is contrary to the teaching of U.S. Pat. No. 4,374,664 which seeks to have aluminum present.
In practice, this implies that the blend quality should ascertain an intimate closeness of the lime particles with the aluminum metal bearing particles so as to guarantee this same location requirement. However, prior to this invention, these lime-aluminum blends, even with all the other additives considered so far such as alumina, have not to our knowledge, been able to compete effectively with lime-magnesium blends or with calcium carbide and/or calcium carbide/magnesium combinations with or without lime.